1. Field of the Invention
This invention relates to differential phase-shift keying (DPSK) in telecommunication. More specifically, it relates to DPSK devices for multiple free-spectral-range operation.
2. Description of the Prior Art
Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Any digital modulation scheme uses a finite number of distinct signals to represent digital data. In the case of PSK, a finite number of phases is used. Each of these phases is assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal (such a system is termed coherent).
Alternatively, instead of using bit patterns to set the phase of the wave, the patterns are used to set changes in the phase of the signal. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK because there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (i.e., it is a non-coherent scheme).
In telecommunication technology, differential phase-shift keying utilizes a decoding method in order to convert the phase-keyed signal to an intensity-keyed signal at the receiving end. The decoding method can be achieved by comparing the phase of two sequential bits. In principle, it splits the input signal beam into two channels with a small delay before recombining them. After the recombination, the beams from the two channels interfere constructively and destructively. The interference intensity is measured and becomes the intensity-keyed signal. To achieve this, one channel has an optical path longer than the other by a distance equivalent to the photon flight time of one bit. For instance, in a 40 Gbit-per-second system, one bit is equal to 25 ps and light travels 7.5 mm in that period. Thus, in this example, the optical path difference (OPD) between the two channels would be set at 7.5 mm.
The Mach-Zehnder type interferometer with a desired OPD between the two channels has been used for decoding purposes. Because of the properties of optical interference, a change in OPD can greatly affect interference intensity. Moreover, the optical path in each arm is much longer than its difference. Therefore, a sophisticated temperature control is required to maintain the optical path in each arm in order to assure that the change in the OPD is much less than a small fraction of one wavelength, e.g., about 10 nm. This is difficult and expensive to achieve, especially for an interferometer with a long optical path.
FIG. 1 shows the typical configuration of a Michelson interferometer 10. The core of such device consists of a beam-splitter cube 12 and two mirrors 14,16 positioned so as to reflect the transmission and reflection beams produced by the beam splitter. Two cavity structures are shown in the figures because commonly used in the art, but it is understood that the mirrors 14,16 are the only relevant optical elements to the Michelson configuration and the rest of the structure can take different forms. The input signal I impinges on the beam-splitting surface 18 of the beam-splitter cube 12 and produces two beams (a reflected beam R and a transmitted beam T), each carrying 50% of the total power. After both beams R,T are reflected by the mirrors 14,16 along their respective optical paths, they return to the beam-splitting surface 18 and are split again, thereby producing two pairs of beams propagating in different directions. The two beams in each pair interfere, both constructively and destructively, to produce two output beams O1 and O2 that are 180 degrees out of phase.
As mentioned, the mirrors 14,16 are illustrated as components of cavity structures formed by combining two pieces of fused silica glass plates (20,22 and 24,26) and a number of spacers (28 and 30) interposed between them. Therefore, the cavity length is defined by the length of the spacers in the cavity. (Two spacers are illustrated for each cavity, but different numbers are often used in the art to meet other design requirements.) The spacers are typically made of a Zerodur® or a ULE® (ultra low expansion) glass, both substances with a coefficient of thermal expansion (CTE) less than 0.05 ppm (i.e., very close to zero), in order to make the free spectral range of the device practically insensitive to temperature variations. The rest of the parts in the interferometer are made of fused silica, which has CTE of 0.5 ppm. The joints between the spacers, the fused silica and the beam splitter are implemented by optical contact technology. In essence, the parts are super-polished, contacted at room temperature, and then thermally treated for several hours in order to obtain a permanent bond. Mirror coatings are used in conventional manner to form the mirrors 14,16 on the glass plates 22,26 to reflect the impinging beams at the distal end of each cavity.
Assuming that the anti-reflection (AR) coated surfaces 32,34 of the glass plates 20,24 at the proximal end of each cavity are equidistant from the beam-splitting surface 18, the lengths L1 and L2 of the cavities determine the optical path difference (OPD) between the two arms of the interferometer 10. As is well understood in the art, the difference in the optical lengths of the two arms (which under the conditions described is the same as the difference between L1 and L2) determines the free spectral range (FSR) of the device. The relationship between the FSR of the device and the cavity lengths is given by
                    FSR        =                              C                          2              ⁢                              (                                                      L                    1                                    -                                      L                    2                                                  )                                              =                      C                          2              ⁢                              (                                                      L                    2                                    -                                      L                    1                                                  )                                                                        (        1        )            where C is the speed of light. For example, an OPD of 3,000 μm will produce an FSR of 100-GHz.
For optical communication DPSK demodulator applications, the relative lengths of the cavities are also selected so as to provide a time delay (τDLI) between the two beams R and T returning to the beam-splitting surface 18 that is close to the time interval (τ0) between two adjacent bits. (If τDLI<τ0, the interferometer can be used as a PDPSK—partial DPSK—demodulator.) FIG. 2 shows the spectra of two outputs O1 and O2 of a DPSK demodulator with FSR=50 GHz.
Copending U.S. application Ser. No. 11/360,959 and No. 11/485,653 describe various embodiments of novel Michelson-type interferometers used as DPSK demodulators. These demodulators provide a significant improvement over the prior art, but the operation of each device is limited to a single free spectral range. The present invention provides a solution for expanding the use of any free-space delay line interferometer (DLI) to multiple free-spectral-range applications.